1. Field of the Invention
The present invention relates to a piezoelectric actuator, and method of producing the same, wherein a reliable electrical contacting design is employed which avoids the negative consequences of potentially-arising voltage cracks.
2. Description of the Prior Art
Piezoelectric actuators normally include a plurality of piezoelectric elements arranged in a stack. Each of these elements, in turn, includes a piezoceramic layer which is provided on both sides with metallic electrodes. If a voltage is applied to these electrodes, the piezoceramic layer reacts with a lattice distortion which leads to a usable lengthwise expansion along a major axis. Since this amounts to less than two parts per thousand of the layer thickness along the major axis, a correspondingly higher layer thickness of active piezoceramic must be provided in order to achieve a desired absolute lengthwise expansion. With increasing layer thickness of the piezoceramic layer within one piezoelectric element, however, the voltage necessary for the response of the piezoelectric element also rises. In order to keep this within manageable limits, multilayer actuators are produced in which the thicknesses of individual piezoelectric elements normally lie between 20 and 200 .mu.m. A piezoelectric actuator must therefore have an appropriate number of individual elements or layers for a desired lengthwise expansion.
Known piezoelectric actuators of multilayer design, therefore, include a total of up to several hundred individual layers. In order to produce them, piezoceramic green films are arranged alternately with electrode material to form a stack. Thereafter, they are laminated and sintered together to form a monolithic composite of up to about 5 mm in height. Larger actuators having a larger absolute deflection can be obtained, for example, by bonding a plurality of such stacks. Adequately high stiffnesses, in particular when high forces have to be transmitted with the piezoelectric actuator, are possessed by piezoelectric actuators of completely monolithic multilayer design, which exhibit an adequately solid composite of the individual layers in the stack.
In order to make electrical contact with such piezoelectric actuators of multilayer design, for example, metallization strips are applied to the outside of the piezoelectric actuator or else in a hole in the center of the surface of the individual actuators. In order for one electrode layer to be used as the electrode for the two adjacent piezoceramic layers, the electrical contacting of the electrode layers within the stack is carried out with alternating polarity. For example, in order to connect each second electrode layer to one of the metallization strips, the latter must be insulated from the electrode layers lying in between. This is brought about in a simple way wherein each second electrode layer has, in the region of the one metallization strip, a cut-out in which it is not led up to the metallization strip. The remaining electrode layers then have the cut-outs in the region of the second metallization strip in order to make contacting possible with alternating polarity.
A further possibility for such alternating contacting consists in insulating each second electrode layer in the region of the metallization strips. This is brought about, for example, by means of glass insulations which, following the production of the stacked monolithic piezoelectric actuator, are applied at the edge of the electrode layers; for example, electrophoretically. However, this process is expensive and is restricted to piezoelectric actuators whose individual ceramic layers have a thickness of at least 100 .mu.m. Because of the low glass insulation path, piezoelectric actuators that are contacted in this way are typically not suitable for either high reliability or unprotected ambient conditions.
Piezoelectric actuators whose alternating contacting is carried out via cut-outs in the electrode layers are piezoelectrically inactive in the contacting region, since it is not possible for any electric field to build up there, as a result of one electrode being missing in each case. This has the result, both in the polarization and during the operation of the piezoelectric actuator, that mechanical stresses build up in this piezoelectrically inactive contacting region. This result which can lead to cracks on the metallization strips parallel to the electrode layers. This can further lead to the complete severing of the metallization strips and has the consequence that, in the case of a point-like voltage supply from the outside to the metallization strips, a part of the piezoelectric actuator becomes uncoupled from the voltage supply. Hence, it becomes inactive. The number of cracks depends on the overall height of the actuator and on the strength of the boundary surface between inner electrode and piezoceramic. The number of cracks may rise further in continuous operation, given alternating load conditions. A crack opening that already exists at the time of polarization is further enlarged parallel to the driving of the actuator. When the voltage is switched off, however, it returns once more to the initial value. Therefore, during dynamic operation, a dynamic change to the cracks or a dynamic change to the crack openings is observed which can further damage the metallization strips.
It is therefore an object of the present invention to produce a piezoelectric actuator which has a reliable electrical contacting design which avoids the negative consequences of potentially arising voltage cracks.